Refactor type_tunnel/gre/vxlan to reduce duplicate code

gre and vxlan type drivers have similar implementation for multiple
methods. This change abstracts these methods and move them to
TunnelTypeDriver.

sync_allocations method is not shared between gre and vxlan type drivers
because they use different strategies and the targeted one is unclear.

Change-Id: I717b70eb718f7ae82f1bc0c81543b0c438e8bddd
Merge "Refactor type_tunnel/gre/vxlan to reduce duplicate code"